include/v8-sandbox.h

// Copyright 2024 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef INCLUDE_V8_SANDBOX_H_
#define INCLUDE_V8_SANDBOX_H_

#include <cstdint>

#include "v8-internal.h"  // NOLINT(build/include_directory)
#include "v8config.h"     // NOLINT(build/include_directory)

namespace v8 {

/**
 * A pointer tag used for wrapping and unwrapping `CppHeap` pointers as used
 * with JS API wrapper objects that rely on `v8::Object::Wrap()` and
 * `v8::Object::Unwrap()`.
 *
 * The CppHeapPointers use a range-based type checking scheme, where on access
 * to a pointer, the actual type of the pointer is checked to be within a
 * specified range of types. This allows supporting type hierarchies, where a
 * type check for a supertype must succeed for any subtype.
 *
 * The tag is currently in practice limited to 15 bits since it needs to fit
 * together with a marking bit into the unused parts of a pointer.
 */
enum class CppHeapPointerTag : uint16_t {
  kFirstTag = 0,
  kNullTag = 0,

  /**
   * The lower type ids are reserved for the embedder to assign. For that, the
   * main requirement is that all (transitive) child classes of a given parent
   * class have type ids in the same range, and that there are no unrelated
   * types in that range. For example, given the following type hierarchy:
   *
   *          A     F
   *         / \
   *        B   E
   *       / \
   *      C   D
   *
   * a potential type id assignment that satistifes these requirements is
   * {C: 0, D: 1, B: 2, A: 3, E: 4, F: 5}. With that, the type check for type A
   * would check for the range [0, 4], while the check for B would check range
   * [0, 2], and for F it would simply check [5, 5].
   *
   * In addition, there is an option for performance tweaks: if the size of the
   * type range corresponding to a supertype is a power of two and starts at a
   * power of two (e.g. [0x100, 0x13f]), then the compiler can often optimize
   * the type check to use even fewer instructions (essentially replace a AND +
   * SUB with a single AND).
   */

  kDefaultTag = 0x7000,

  kZappedEntryTag = 0x7ffd,
  kEvacuationEntryTag = 0x7ffe,
  kFreeEntryTag = 0x7fff,
  // The tags are limited to 15 bits, so the last tag is 0x7fff.
  kLastTag = 0x7fff,
};

// Convenience struct to represent tag ranges. This is used for type checks
// against supertypes, which cover a range of types (their subtypes).
// Both the lower- and the upper bound are inclusive. In other words, this
// struct represents the range [lower_bound, upper_bound].
// TODO(saelo): reuse internal::TagRange here.
struct CppHeapPointerTagRange {
  constexpr CppHeapPointerTagRange(CppHeapPointerTag lower,
                                   CppHeapPointerTag upper)
      : lower_bound(lower), upper_bound(upper) {}
  CppHeapPointerTag lower_bound;
  CppHeapPointerTag upper_bound;

  // Check whether the tag of the given CppHeapPointerTable entry is within
  // this range. This method encodes implementation details of the
  // CppHeapPointerTable, which is necessary as it is used by
  // ReadCppHeapPointerField below.
  // Returns true if the check is successful and the tag of the given entry is
  // within this range, false otherwise.
  bool CheckTagOf(uint64_t entry) {
    // Note: the cast to uint32_t is important here. Otherwise, the uint16_t's
    // would be promoted to int in the range check below, which would result in
    // undefined behavior (signed integer undeflow) if the actual value is less
    // than the lower bound. Then, the compiler would take advantage of the
    // undefined behavior and turn the range check into a simple
    // `actual_tag <= last_tag` comparison, which is incorrect.
    uint32_t actual_tag = static_cast<uint16_t>(entry);
    // The actual_tag is shifted to the left by one and contains the marking
    // bit in the LSB. To ignore that during the type check, simply add one to
    // the (shifted) range.
    constexpr int kTagShift = internal::kCppHeapPointerTagShift;
    uint32_t first_tag = static_cast<uint32_t>(lower_bound) << kTagShift;
    uint32_t last_tag = (static_cast<uint32_t>(upper_bound) << kTagShift) + 1;
    return actual_tag >= first_tag && actual_tag <= last_tag;
  }
};

constexpr CppHeapPointerTagRange kAnyCppHeapPointer(
    CppHeapPointerTag::kFirstTag, CppHeapPointerTag::kLastTag);

class SandboxHardwareSupport {
 public:
  /**
   * Initialize sandbox hardware support. This needs to be called before
   * creating any thread that might access sandbox memory since it sets up
   * hardware permissions to the memory that will be inherited on clone.
   */
  V8_EXPORT static void InitializeBeforeThreadCreation();
};

namespace internal {

#ifdef V8_COMPRESS_POINTERS
V8_INLINE static Address* GetCppHeapPointerTableBase(v8::Isolate* isolate) {
  Address addr = reinterpret_cast<Address>(isolate) +
                 Internals::kIsolateCppHeapPointerTableOffset +
                 Internals::kExternalPointerTableBasePointerOffset;
  return *reinterpret_cast<Address**>(addr);
}
#endif  // V8_COMPRESS_POINTERS

template <typename T>
V8_INLINE static T* ReadCppHeapPointerField(v8::Isolate* isolate,
                                            Address heap_object_ptr, int offset,
                                            CppHeapPointerTagRange tag_range) {
#ifdef V8_COMPRESS_POINTERS
  // See src/sandbox/cppheap-pointer-table-inl.h. Logic duplicated here so
  // it can be inlined and doesn't require an additional call.
  const CppHeapPointerHandle handle =
      Internals::ReadRawField<CppHeapPointerHandle>(heap_object_ptr, offset);
  const uint32_t index = handle >> kExternalPointerIndexShift;
  const Address* table = GetCppHeapPointerTableBase(isolate);
  const std::atomic<Address>* ptr =
      reinterpret_cast<const std::atomic<Address>*>(&table[index]);
  Address entry = std::atomic_load_explicit(ptr, std::memory_order_relaxed);

  Address pointer = entry;
  if (V8_LIKELY(tag_range.CheckTagOf(entry))) {
    pointer = entry >> kCppHeapPointerPayloadShift;
  } else {
    // If the type check failed, we simply return nullptr here. That way:
    //  1. The null handle always results in nullptr being returned here, which
    //     is a desired property. Otherwise, we would need an explicit check for
    //     the null handle above, and therefore an additional branch. This
    //     works because the 0th entry of the table always contains nullptr
    //     tagged with the null tag (i.e. an all-zeros entry). As such,
    //     regardless of whether the type check succeeds, the result will
    //     always be nullptr.
    //  2. The returned pointer is guaranteed to crash even on platforms with
    //     top byte ignore (TBI), such as Arm64. The alternative would be to
    //     simply return the original entry with the left-shifted payload.
    //     However, due to TBI, an access to that may not always result in a
    //     crash (specifically, if the second most significant byte happens to
    //     be zero). In addition, there shouldn't be a difference on Arm64
    //     between returning nullptr or the original entry, since it will
    //     simply compile to a `csel x0, x8, xzr, lo` instead of a
    //     `csel x0, x10, x8, lo` instruction.
    pointer = 0;
  }
  return reinterpret_cast<T*>(pointer);
#else   // !V8_COMPRESS_POINTERS
  return reinterpret_cast<T*>(
      Internals::ReadRawField<Address>(heap_object_ptr, offset));
#endif  // !V8_COMPRESS_POINTERS
}

}  // namespace internal
}  // namespace v8

#endif  // INCLUDE_V8_SANDBOX_H_